Grow Lighting Distance

by Mark
(North Carolina)

I'm using "My Best Hydroponic System" and would like to know how far above the lights should be for optimum performance. I have 2 400 watt HPS lights back to back, but only have ten plants in the middle of that 36 plant system. Needless to say I am a first time grower and would appreciate any insight you could give me. Thanks for your patients with us beginners.

Answer: Mark- I would position the 400 watt lights 12 to 18 inches from the tops of your plants to the bottom of your lights. The hydroponic system you are using is 4' x 8'. Imagine the space as two 4' x 4' areas side by side. One light should be positioned in the center of each 4' x 4' area. Your biggest concern should be the temperature at the tops of the plants directly underneath each light.

An indoor/outdoor thermometer with a remote probe is an invaluable tool for this, and some brands (like Sunleaves) cost as little as $25.00. The next thing you want to pay attention to is the temperature of the rest of the air in the grow room- keep it as close to 70 degrees as possible. Usually, the farther above 70 the temperature goes, the more problems you will have with your garden (especially if mites or other indoor garden pests were present to begin with). With an indoor/outdoor thermometer, the indoor part can keep track of the grow room temperature while you place the "outdoor" probe in your hot spot (directly beneath a light at plant level).

My only other suggestion would be: if you are only growing ten plants, and they are relatively small plants in nature, you should be able to use just one 4' x 4' side of the hydroponic system (half of the system) and only need to use one of your two 400 watt lights. The key is having your plants fit into the 4' x 4' area, because that is the very maximum area a 400 watt light can illuminate effectively when gardening sun-loving plants (which would include most garden veggies, herbs, and flowers). Hope this helps you out, and Happy Growing!